Catalyst carrier

ABSTRACT

The present invention relates to a catalyst carrier characterized by the fact that it comprises a substance which promotes a transition of an alumina that can be converted to α-alumina, which substance is located near the surface of a carrier composed mainly of alumina and which carrier is subjected to a heat treatment under appropriate conditions, thereby causing the transition of said alumina to α-alumina, with those pores near the surface made larger in diameter than those pores located more deeply within the carrier. The invention also includes a method of manufacturing said catalyst carrier.

This is a continuation of application Ser. No. 568,550, filed Apr. 16,1975, now U.S. Pat. No. 4,039,481.

BACKGROUND OF THE INVENTION

Generally a catalyst is evaluated not only in terms of its activity butalso in terms of its useful life. A catalyst, even though excellent inactivity, is practically useless if it has a short life. It is alsouseless if it is easily breakable and therefore short in mechanicallife.

Usually for the manufacture of a high-activity, long-life catalyst, acatalyst carrier is required to meet many conditions. For instance, tobe usable in an exhaust gas purifying catalyst device for renderingharmful elements contained in auto engine emission gases such as carbonmonoxide, unburnt hydrocarbons and nitrogen oxides harmless, thecatalyst is required to meet the following conditions.

First, since the temperature in the engine during combustion becomeshigher than 800° C., the catalyst used must be one that exhibits only asmall decrease in activity even at high temperatures. Besides, it mustbe strong enough to withstand the vibrational forces generated in autooperation.

Automotive fuel containing chemical compounds such as lead, sulphur andphosphorus naturally produces exhaust gases containing lead compounds,sulphur compounds and phosphorus compounds. It is equally natural thatsince the lubricating oil for the internal combustion engine containingphosphorus compounds or the like which is, in most cases, burned in theinternal combustion engine, the exhaust gas also contains elementsattributable to the lubricating oil. Among others, the lead compounds,the sulphur compounds and the phosphorus compounds are commonly foundharmful to the catalyst. Therefore the catalyst is required to have anadequate resistance to these harmful substances.

Generally speaking, the mechanical strength and the above-mentionedresistance to the harmful substances contained in the exhaust gas dependlargely on the carrier for the catalyst. For this reason the carrier forthe exhaust gas purifying catalyst has to be strong enough to resistboth thermal shock and mechanical vibration and also sufficientlyresistant to the harmful contents of the exhaust gas.

SUMMARY OF THE INVENTION

The present invention provides a catalyst carrier which is mechanicallyas strong as the conventional carrier and sufficiently resistant to theharmful contents of the exhaust gas, said catalyst carrier consisting ofalumina or being mainly composed of alumina with the alumina grains nearthe surface being larger so that the pores between said grains near thesurface have a larger diameter than those pores located more deeplywithin the carrier.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 shows the distribution of pores in carrier D of Example 2,according to the present invention, in a conventional carrier A, and inthe carrier C of Comparative Example 1.

FIG. 2 is an enlarged sectional view for illustrating the carrieraccording to the present invention wherein (a) is Fe compound-containinglayer and (b) is a layer which does not contain Fe compound.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to a catalyst carrier and to a method ofmanufacturing it. Alumina is the material most commonly used as thecarrier for the catalyst used in the purification of automobile engineemissions.

So far, many proposals have been made with respect to an alumina carriercomposed mainly of alumina and an active alumina, but they have failedto provide satisfactory solutions to the above-mentioned problem.

For instance, Smith et al, in the specification of their U.S. Pat. No.2,422,172, state that oxides of Cr, Mn, Fe, Mo and Co can promote thethermal transformation of γ-alumina into α-alumina. Furthermore, theysuggest the reaction of active aluminas with a certain kind of alkalineearth compound to suppress this tendency for active alumina to betransformed into a phase of higher density by thermal transformation. Alarge number of similar proposals and studies have been made. Forexample, the specification of Japanese Patent Applications laid open tothe public under the numbers Sho 48-14600 and Sho 48-17467 say that, byreaction with a certain kind of rare earth compound, active aluminas canbe prevented from turning into a phase of higher density.

When, however, the above-mentioned compounds are added to alumina andthe alumina grain growth is accelerated through heat treatment at hightemperatures, for instance when Fe compound is added thereto, the porediameter increases, as indicated in FIG. 1, but this is accompanied witha tendency for the crushing strength of the alumina to decrease, whichresults in poor durability. If an alkaline earth compound or a rareearth compound is added to prevent the tendency of alumina to turn intoa phase of higher density, the decrease in the crushing strength willnot be great. A carrier with its mechanical strength increased by theaddition of such compounds or by a reduction in the pore diameter, whenused to carry an automotive exhaust gas purifying catalyst, will cause agreater decrease in the catalyst performance than a carrier of lowstrength, i.e., one with a large pore diameter. Elaborate, extensiveresearch undertaken by the present inventors has revealed that suchperformance drop, is caused by the harmful contents of the exhaust gas.

From this it follows that in order to increase its mechanical strength,the catalyst carrier has to be composed of fine alumina grains. Thismeans that an alumina with a wider specific surface area is used andthis is effective for increasing the dispersion of the metal carried andthe durability of the catalyst. As stated before, however, when thiskind of carrier is used to carry the catalyst, there is a substantialdrop in the activity of the catalyst due to the harmful contents of theexhaust gas.

After full investigation of carriers for automotive exhaust gaspurifying catalysts, the present inventors have succeeded in inventing acatalyst carrier with ample mechanical strength, which also assuresample resistance of the catalyst to the above-mentioned harmful contentsof the exhaust gas, by making the diameter of those pores near thesurface larger than that of those pores located more deeply inside,thereby increasing the specific surface area of the internal alumina.

The features of the catalyst carrier according to the present inventionare that the conflicting relationship, traced to a carrier which is madeof alumina or consists mainly of alumina, between the crushing strengthand the resistance to the harmful contents of the exhaust gas has beenovercome. Its strength is also satisfactory; ample resistance of thecatalyst to the harmful contents of the exhaust gas is assured; and theheat resistance too is enhanced, thereby assuring a long life for thecatalyst.

The present invention has been brought about by adding to the surface ofthe carrier a compound which can promote the transition of an aluminathat can be converted to α-alumina γ-alumina to α-alumina as a means ofenlarging the diameter of the surface pores by accelerating the aluminagrain growth near the surface through heat treatment under theconditions that permit the transition of the alumina near the surface toα-alumina.

More specifically, the present invention provides a catalyst carrierwith an alumina that can be transformed to α-alumina, such as γ-aluminaas its main component, which contains near its surface a compound whichpromotes the transition to α-alumina, said catalyst carrier being firedunder conditions permitting the transition of surface X- or γ-alumina tothe α-phase with the result that the pore-growth of the surface aluminabecomes faster than that of the internal alumina, so that the diameterof the surface pores becomes larger than that of the internal pores.

In the present invention, Fe compounds are preferable as an additive toalumina which is cheap, low in secondary public hazard and sufficientlyeffective for attaining the object of the present invention, but it isapparent that compounds of Cr, Mn, Mo, Cu, W, Ti, and V which canpromote transition of alumina to α-alumina may also be used.

As the first step toward attaining the above-mentioned carrier, thereare various methods for introducing the Fe compound near the surface ofa carrier of alumina or composed mainly of alumina.

For instance, there is a method for obtaining a carrier easily bycontacting a carrier which has been impregnated in advance with asolution containing ammonia water or hydroxides such as NaOH, KOH, BaOH,carbonates such as Na₂ CO₃, NaHCO₃ or sulfides such as (NH₄)₂ S, H₂ Sand the like or a carrier which has been subjected to drying ifnecessary, with a solution containing Fe compound.

Any Fe compound is suitable so long as it can be deposited near thesurface of the carrier. It may be, for instance, a mineral acid saltsuch as a nitrate or a sulfate; a chloride; a double salt such as Mohr'ssalt and iron alumi; or an organic acid salt such as an acetate.

Numerous other methods can be used for introducing the Fe compound nearthe surface of carrier, for instance, such methods as spraying asolution or gas containing an Fe compound against the carrier, orcoating the alumina carrier with an alumina layer containing an Fecompound.

Further, the inclusion of an Fe compound near the surface of the carrieris not limited to a use of a previously molded carrier. For instance,representative methods for adding Fe compound well-known to personsskilled in the art include: the formation of an alumina gel,agglomeration, extrusion and pelletization, through which alumina can begiven a desired shape or size, or the Fe compound addition may becarried out in such a process of making a carrier as in the process ofdripping in oil.

The present invention can be accomplished by firing an alumina carrieror a carrier mainly composed of alumina, into which an Fe compound hasbeen introduced to a depth of less than 400μ from the surface, underconditions permitting the iron-containing alumina layer to turn into theα-phase.

The Fe component of the carrier according to the present invention isultimately solid-solutioned in the form of Fe⁺⁺, Fe⁺⁺⁺ etc. or ispresent in the form of an oxide.

In the following examples the present invention will be concretelydescribed, but these examples are representative rather than exhaustive.

EXAMPLE 1

A commercially available active alumina carrier (a product of theSumitoma Chemical Industry Co., composed mainly of X alumina;hereinafter called carrier A) was immersed in a 0.5 N ammonia watersolution for 3 minutes, after which excess solution was removed. Thecarrier A was then immersed in an aqueous solution containing ferrouschloride in the proportion of 0.2 mole/l for 5 minutes, followed bywashing with water and drying at 120° C. Next, a 5-hour heat treatmentat 900° C. was carried out in order to produce the desired carrier,which will be called carrier B. The Fe compound-containing layer ofcarrier B extends about 30μ inward from the carrier surface. X-raydiffraction revealed α-alumina in the alumina of the Fecompound-containing layer, but no α-alumina was found in the alumina ofthe deeper layer having no content of Fe compound.

COMPARATIVE EXAMPLE 1

The same carrier A as used in Example 1 was heat-treated at 900° C. for5 hours, yielding the carrier C.

No α-alumina was found in the carrier C.

COMPARATIVE EXAMPLE 2

The same carrier A as was used in Example 1 was immersed in an aqueoussolution containing ferric nitrate in the proportion of 1 mole/l for onehour, followed by the removal of excess solution, drying at 120° C. andthen a 5-hour heat treatment at 900° C., thereby yielding the carrier D.Carrier D was found to contain the Fe compound practically to its core,with α-alumina being revealed by X-ray diffraction.

By introducing mercury under pressure, the distribution of pores wasmeasured in the case of the carriers A, C and D, the results beingsummarized in FIG. 1.

In FIG. 1 the ordinate is the pore volume ratio (%) and the abscissa thepore diameter. The pore volume ratio is calculated as follows:

    Pore volume ratio = Vp/Vt × 100 (%)

where

Vp (in cc) = Volume of mercury introduced up to each measuring point(pore diameter)

Vt (in cc) = Total volume of mercury introduced into the pores at theend of the measurement

From FIG. 1 it is seen that the carrier D which contains an Fe compoundand has been heat-treated tends to have a larger pore diameter becauseof the Fe compound.

The crushing strengths of said carriers A, B, C and D were also measuredwith the results summarized in Table 1.

                  TABLE 1                                                         ______________________________________                                        Carriers       Crushing Strength (Kg/grain)                                   ______________________________________                                        A              12                                                             B               9                                                             C              11                                                             D               3                                                             ______________________________________                                    

From Table 1 it is found that in the carrier B with a controlledthickness of the Fe compound-containing layer, the drop in themechanical strength which occurs when the content of the Fe compoundextends throughout the carrier can be considerably prevented and thecarrier performance can be maintained nearly the same as in aconventional carrier.

Next, an example of practically testing a catalyst carried in thecarrier of the present invention is illustrated.

TEST 1

Carriers A and B were immersed in an aqueous solution acidified bynitric acid, of acidic palladium nitrate-chloroplatinic acid containingPd at a rate of 2 g/l and Pt at a rate of 1 g/l for 20 minutes, followedby the removal of excess solution, drying at 120° C., a 2-hour reductionwith hydrogen at 500° C., repeated washing with water and drying at 120°C., thereby yielding an alumina-carried platinum-palladium catalyst. Thecatalyst obtained using the carrier A is defined as Catalyst A and theone obtained using the carrier B is defined as Catalyst B.

Catalysts A and B were subjected to a durability test with acceleratedexposure to catalyst poisons. This durability test was carried out toinvestigate the resistance of the catalyst to the harmful contents ofthe exhaust gas, using a gasoline containing Pb 0.6g/U.S. gallon and P0.10g/U.S. gallon and a real engine under the following conditions:

    ______________________________________                                        Space velocity   75,000 - 90,000 ml/hr per ml                                                  of catalyst carried.                                         Catalyst bed temperature                                                                       800° C                                                Duration         50 hours                                                     Atmosphere       CO/O.sub.2 = 0.5 - 8%                                                         CO = 1.2% approx.                                            ______________________________________                                    

The results of a propane oxidation activity test and a CO oxidationactivity test carried out with catalysts A and B before and after theabove-mentioned accelerated exposure to catalyst poisons are summarizedin Table 2.

In the present invention the propane oxidation activity test wasinvariably executed under the following conditions:

    ______________________________________                                        Space velocity   18,000 ml/hr per ml of                                                        catalyst.                                                    Reactor temperature                                                                            400° C                                                Supplied gas     C.sub.3 H.sub.8 - 570 ppm                                                     CO - 1.2%                                                                     air - balance                                                Analyzer         Hydrocarbon analyzer                                         ______________________________________                                    

The rate of propane purification was calculated as follows:

    Rate of propane purification = [(X - Y)/X] × 100 (%)

where X . . . initial concentration of propane

Y . . . residual concentration of propane after passing through catalystbed of about 400° C.

In the present invention the CO oxidation activity test was invariablyexecuted under the following conditions:

    ______________________________________                                        Space velocity     30,000 ml/hr per ml of                                                        catalyst carried.                                          Incoming gas temperature                                                                         275° C                                              Supplied gas       CO - 2.5%                                                                     air - balance                                              Analyzer           CO-tester                                                  ______________________________________                                    

The rate of CO purification was calculated as follows:

    Rate of CO purification = [(W - V)/W] × 100 (%)

where W . . . initial concentration of CO

V . . . residual concentration of CO after 8 minutes' gas supply

                  TABLE 2                                                         ______________________________________                                        Rate of propane     Rate of CO                                                Purification (%)    Purification (%)                                                  Before    After     Before  After                                             durability                                                                              durability                                                                              durability                                                                            durability                                Catalysts                                                                             test      test      test    test                                      ______________________________________                                        A       96        42        100     67                                        B       96        70        100     94                                        ______________________________________                                    

Table 2 shows that the catalyst B using the carrier B of the presentinvention shows a higher rate of purification after the test than thecatalyst A using the carrier A for comparison.

Putting together the results of the tests summarized in Tables 1 and 2,it appears that the carrier of the present invention is mechanically asstrong as the conventional carrier and has a higher resistance to theharmful contents of the exhaust gas than the conventional carrier.

EXAMPLE 2

A commercially available active alumina carrier (a product ofRhone-Prozil; composed mainly of γ-alumina; hereinafter called thecarrier E) was immersed in a 0.4 N ammonia water solution of 3% hydrogenperoxide for 5 minutes, after which excess solution was removed from thecarrier E. This was followed by a 5-minute immersion in an aqueoussolution of ammonium ferrous sulfate at a rate of 0.5 mole/l, washingwith water, drying at 120° C. and then a 2-hour heat treatment at 1100°C., thereby yielding the desired carrier, which will be called carrierF. The Fe compound-containing layer in the carrier F extended to a depthof about 110μ from the surface, with most of the Fe-containing aluminaturned into α-alumina; in the deep part containing no Fe compound,however, only a little α-alumina was found.

EXAMPLE 3

After the heat treatment in Example 2 was carried out at 1000° C. for 2hours, a desired carrier was produced which will be called carrier G.The Fe compound-containing layer of the carrier G was extended about asfar inward as in the carrier F, that is, it reached a depth of about110μ. α-alumina was found in the alumina of the Fe-compound containinglayer, but practically none was found in the alumina of deeper layers.

EXAMPLE 4

After the heat treatment in Example 2 was carried out at 1100° C. for 5hours, a desired carrier was produced which will be called carrier H.The Fe compound-containing layer of carrier H was nearly the same as incarrier F, that is, it reached a depth of about 110μ from the surface.Practically all alumina in the Fe compound-containing layer turned outto be α-alumina and the alumina in the deeper layers was also α-alumina.

EXAMPLE 5

After treatment similar to Example 2 with the normality of the ammoniawater solution containing hydrogen peroxide set at 0.5 N and the heattreatment carried out at 1100° C. for 1 hour, a desired carrier wasproduced, which will be called carrier I.

The Fe compound-containing layer of the carrier I extended to a depth ofabout 50μ from the surface, and practically all alumina in the Fecompound-containing layer turned out to be α-alumina. Even in the deeperlayer containing no Fe compound, α-alumina was found.

The measured crushing strength of the carriers E, F, G, H and I arelisted in Table 3.

                  TABLE 3                                                         ______________________________________                                                         Crushing Strength                                            Carriers         (Kg/grain)                                                   ______________________________________                                        E                7                                                            F                8                                                            G                8                                                            H                7                                                            I                7                                                            ______________________________________                                    

From Table 3 it is found that the carrier according to the presentinvention is equal in mechanical strength to a conventional carrier.

Using these carriers, in the same way as in Test 1, the propaneoxidation activity and the CO oxidation activity before and after theaccelerated exposure to catalyst poisons were measured.

TEST 2

Catalysts E to I were produced by impregnating the carriers E, F, G, Hand I with palladium to a depth of 200μ from the surface by usingpalladium chloride. Carriers and catalysts are matched in code, with thecarrier E matching the catalysts E, for example.

The results of a propane oxidation activity test and a CO oxidationactivity test carried out with catalysts E, F, G, H and I before andafter the accelerated exposure to catalyst poisons are summarized inTable 4.

                  TABLE 4                                                         ______________________________________                                                   Rate of propane                                                                           Rate of CO                                                        purification (%)                                                                          purification (%)                                                        Before   After  Before After                                         Carriers durability                                                                             durability                                                                           durability                                                                           durability                            Catalysts                                                                             used     test     test   test   test                                  ______________________________________                                        E       E        95       30     99     32                                    F       F        97       69     99     97                                    G       G        96       65     99     91                                    H       H        97       72     99     96                                    I       I        99       70     99     97                                    ______________________________________                                    

Table 4 shows that the catalysts corresponding to the carriers F, G, Hand I of the present invention excel the compared carrier E with respectto these rates of purification after the test.

Summing up the results in Tables 3 and 4, it is found that the carrieraccording to the present invention is comparable in mechanical strengthto a conventional carrier, but with respect to its resistance to theharmful contents of exhaust gas, it is superior to a conventionalcarrier, eliminating the latter's short-comings.

Next, the results of the percentages for Fe compound content and formetal catalyst with regard to each of the catalyst used in Tests 1 and 2are tabulated in Table 5.

                  Table 5                                                         ______________________________________                                        Carrier used                                                                             Catalyst                                                           Fe content        Fe content                                                                              Pd content                                                                            Pt content                                (%)               (%)       (%)     (%)                                       ______________________________________                                        A    --        A      --      0.18    0.09                                    B    0.13      B      0.13    0.18    0.09                                    E    --        E      --      0.12    --                                      F    0.18      F      0.18    0.12    --                                      G    0.18      G      0.18    0.12    --                                      H    0.17      H      0.17    0.12    --                                      I    0.12      I      0.12    0.12    --                                      ______________________________________                                    

Further, to explain about the carrier according to the presentinvention, FIG. 2 is a cross-section of the carrier cut through thecentre thereof in order to illustrate the carrier of the presentinvention.

In the figure, (a) shows a Fe compound-containing layer while (b) showsa layer which does not contain Fe compound. The present inventioncomprises having the pore diameter of the layer (a) larger than that ofthe layer (b).

                  Table 6                                                         ______________________________________                                                Pore diameter (μ) at 50%                                                   integrated pore volume ratio                                          Carrier   (a) layer    (b) layer    Average                                   ______________________________________                                        B         0.03         0.01         0.02                                      F         0.14         0.10         0.11                                      G         0.07         0.03         0.04                                      H         0.17         0.12         0.14                                      I         0.07         0.03         0.04                                      ______________________________________                                    

From Table 6, it is seen that all the carriers according to the presentinvention have larger pore diameter in the layer (a) than the layer (b)at a 50% integrated pore volume ratio.

What is claimed is:
 1. Catalyst carrier having a surface layerconsisting mainly of α-phase alumina, and an inner portion consistingmainly of alumina that is not α-alumina, which has pores in theα-alumina surface layer which are larger than those in said innerportion.
 2. The catalyst carrier of claim 1 in which said surface layercontains an iron-containing compound, but said inner portion issubstantially iron-free.
 3. The catalyst carrier of claim 1, in whichsaid surface layer also contains a compound which promotes thetransition of alumina to α-alumina selected from the group consisting ofFe, Cr, Mn, Mo, Cu, W, Ti and V compounds.